Various kinds of rolling bearings are used in the rotation support sections of various kinds of mechanical devices. Of these rolling bearings, four-point contact ball bearings in which the rolling surfaces of each of the balls come in contact with an inner raceway and outer raceway at two points each for a total of four contact points are able to ensure a load capacity to a certain extent not only to radial loads but also to axial loads, and when compared with a typical deep-groove ball bearing, can be constructed compactly and the dynamic torque (rotation resistance) can be kept low. FIG. 5 illustrates an example of a conventional four-point contact ball bearing such as disclosed in JP 2005-188686 (A), JP 2007-218368 (A) and the like. The four-point contact ball bearing 1 comprises an inner ring 2 and outer ring 3 that are arranged concentric with each other, and plural balls 4. The inner ring 2 has an inner raceway 5 around the outer-circumferential surface thereof, and the outer ring 3 has an outer raceway 6 around the inner-circumferential surface thereof.
Both the inner raceway 5 and the outer raceway 6 have a gothic arch shaped generating line shape. In other words, the inner raceway 5 is a complex curved surface with a pair of inner raceway side surface sections 7 that have a generating line shape with a radius of curvature that is larger than 50% the diameter of the balls 4, and an inner raceway bottom section 8 that is located in the center section in the width direction of the inner raceway 5 and connects the inside end edges in the width direction of the inner raceway side surface sections 7. The outer raceway 6 also is a complex curved surface with a pair of outer raceway side surface sections 9 that have a generating line shape with a radius of curvature that is larger than 50% the diameter of the balls 4, and an outer raceway bottom section 10 that is located in the center section in the width direction of the outer raceway 6 and connects the inside end edges in the width direction of the outer raceway side surface sections 9. The balls 4 are held by a retainer 11 so as to roll freely, and are arranged so as to roll freely between the inner raceway 5 and outer raceway 6.
In the four-point contact ball bearing 1, the inner raceway 5 and outer raceway 6 and the rolling surface of each of the balls 4 come in rolling contact at two points each for a total of four points. In this kind of four-point contact ball bearing 1, an inner-diameter-side member such as a rotating shaft around which the inner ring 2 is fastened relatively rotates on the inner-diameter side of an outer-diameter-side member such as a housing inside which the outer ring 3 is fastened, and this kind of four-point contact ball bearing 1 is not only able to support radial loads that act between the inner ring 2 and outer ring 3, but is also able to support axial loads that act between the inner ring 2 and outer ring 3.
The four-point contact ball bearing 1 illustrated in FIG. 5 takes into consideration use in a pulley for an auxiliary device for engine such as an alternator, so comprises a pair of seal rings 12 for closing off the openings on both ends of the space 13 inside the bearing where the balls 4 are located, and preventing grease that is filled inside the space 13 inside the bearing from leaking, and preventing foreign matter that is located in the external space from getting inside the space 13 inside the bearing. However, in the case of placing the four-point contact ball bearing 1 inside a housing of an automobile transmission, the seal rings may be omitted, and flow of lubrication oil that flows in the space inside the bearing may be regulated by providing a non-contact type shield ring.
In the four-point contact ball bearing 1, the portions of the pair of inner raceway side surfaces sections 7 and the pair of outer raceway side surface sections 9 that come in contact with the rolling surfaces of the balls 4 during the normal operating state are processed and finished by super finishing and have smooth surfaces. On the other hand, finishing is not performed for the inner raceway bottom section 8, the outer raceway bottom section 10, the portions of the pair of inner raceway side surface sections 9 that are near the inner raceway bottom section 8, and the portions of the pair of outer raceway side surface sections 7 that are near the outer raceway bottom section 10. Therefore, as illustrated in FIG. 6, the surfaces of the inner raceway bottom section 8, outer raceway bottom section 10 and portions near these sections are rougher than the portions of the pair of inner raceway side surface sections 7 and outer raceway side surface sections 9 that come in contact with the rolling surfaces of the balls 4 during the normal operating state. The reason for this is that, as illustrated in FIG. 5, the inner raceway bottom section 8 and the outer raceway bottom section 10 are angled sections having large curvature (small radius of curvature), or relief grooves are formed, so it is difficult to bring a finishing grindstone into contact with these portions.
When the four-point contact ball bearing 1 is used in the rotation support section of a pulley for auxiliary device for engine, the radial loads that are applied to the four-point contact ball bearing 1 are limited, and the rolling surfaces of the balls 4 do not come in contact with the inner raceway bottom section 8, outer raceway bottom section 10 and the portions near these sections, so no particular problems occur even when finishing is not performed for these portions.
However, when the four-point contact ball bearing 1 is assembled in the rotation support section of an automobile drive system such as a power transmission shaft inside a transmission, there is a possibility that problems may occur from the aspect of maintaining durability of the ball bearing. For example, force in the radial direction is applied to the four-point contact ball bearing 1 from the power transmission shaft inside a transmission due to a gear reaction force that occurs in the engaging section of the power transmission gear. The size of the radial force due to this kind of gear reaction force does not become large when a vehicle is traveling at constant speed, however, when the vehicle accelerates suddenly, or when a large engine brake is obtained in a low gear during high-speed operation, the force becomes considerably large.
In this case, there is a possibility, even if for only a short period of time, that the rolling surfaces of the balls 4 will come in contact with the unfinished rough surfaces of the inner raceway bottom section 8, outer raceway bottom section 10 and the portions near these sections, or when relief grooves are formed, there is a possibility that the rolling surfaces of the balls 4 will have so-called edge contact with the edges of the relief grooves. When the rolling surfaces of the balls 4 come in contact with the rough surfaces or the edges of the relief grooves, the rolling surfaces of the balls 4 will become damaged and the rolling fatigue life of the four-point contact ball bearing will be shortened. Particularly, in the inner raceway 5 having a convex arc shape in the circumferential direction, the surface pressure is higher at the areas of contact with the rolling surfaces of the balls 4 than in the outer raceway 6 having a concave arc shape in the circumferential direction, so when the balls 4 coming in contact with the inner raceway bottom section 8 or portions near that section, it becomes even easier for the rolling surfaces of the balls 4 to become damaged.
JP 2002-039190 (A) discloses performing super finishing up to the bottom section of this raceway surface in order to suppress a decrease in the durability of a four-point contact ball bearing due to a cause such as this, with the generating line shape of the raceway surface being an elliptical shape, a parabolic shape or a hyperbolic shape. However, in the case of this construction, the contact angle of the balls becomes large, the radial load capacity of the four-point contact ball bearing becomes low, and when an axial load is applied, it becomes easy for the rolling surfaces of the balls to ride up on the shoulder section of the raceway surface. Therefore, with this construction, in uses in which large radial loads and axial loads are applied, it is not always possible to sufficiently maintain the durability of a four-point contact ball bearing.
JP 2006-118591 (A) discloses construction in which the shapes of the inner raceway and outer raceway are asymmetrical, and that is capable of supporting large axial loads, however, it is not possible to suppress a decrease in durability of a four-point contact ball bearing due to causes such as described above.